UK SMI ID 9 · Trainer
Identification of Actinomyces
An interactive drill, not a textbook. Built to make you retrieve the few facts that actually get tested, not re-read the whole table.
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The shape of the organism core fact
Click the card to flip. This is the one-line description you should be able to recite without thinking.
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Answer
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Nomenclature — what you actually need generalised
Memorise this sentence, not the table
Taxonomy has been revised: many former Actinomyces species have been reclassified into new genera (Schaalia, Winkia, Gleimia, Pauljensenia, Bowdeniella). The species name you learned may not be the current valid name — check before reporting.
If you want a couple of concrete anchors, these are the ones worth keeping:
| Still called Actinomyces | Renamed |
|---|---|
| A. israelii (the classic cause of actinomycosis) | A. odontolyticus → Schaalia odontolytica |
| A. naeslundii | A. meyeri → Schaalia meyeri |
| A. viscosus | A. neuii → Winkia neuii |
Full reassignment table (27 species) — for reference, not for rote memorisation:
Show full table
| Previous name | Current name |
|---|---|
| Actinomyces israelii | Actinomyces israelii |
| Actinomyces graevenitzii | Actinomyces graevenitzii |
| Actinomyces gerencseriae | Actinomyces gerencseriae |
| Actinomyces naeslundii | Actinomyces naeslundii |
| Actinomyces odontolyticus | Schaalia odontolytica |
| Actinomyces viscosus | Actinomyces viscosus |
| Actinomyces funkei | Schaalia funkei |
| Actinomyces europaeus | Gleimia europaea |
| Actinomyces urogenitalis | Actinomyces urogenitalis |
| Actinomyces meyeri | Schaalia meyeri |
| Actinomyces neuii | Winkia neuii |
| Actinomyces neuii subsp. neuii | Winkia neuii subsp. neuii |
| Actinomyces neuii subsp. anitratus | Winkia neuii subsp. anitrata |
| Actinomyces radingae | Schaalia radingae |
| Actinomyces turicensis | Schaalia turicensis |
| Actinomyces radicidentis | Actinomyces radicidentis |
| Actinomyces cardiffensis | Schaalia cardiffensis |
| Actinomyces oricola | Actinomyces oricola |
| Actinomyces nasicola | Bowdeniella nasicola |
| Actinomyces massiliensis | Actinomyces massiliensis |
| Actinomyces johnsonii | Actinomyces johnsonii |
| Actinomyces dentalis | Actinomyces dentalis |
| Actinomyces hongkongensis | Pauljensenia hongkongensis |
| Actinomyces hominis | Gleimia hominis |
| Actinomyces oris | Actinomyces oris |
| Actinomyces timonensis | Actinomyces timonensis |
| Actinomyces georgiae | Schaalia georgiae |
Clinical anchor points
| Fact | Detail |
|---|---|
| Where it lives normally | Oral cavity, GI tract, female urogenital tract (normal microbiota) |
| Main cause of actinomycosis | Actinomyces israelii |
| Other reported causes | A. odontolyticus, A. meyeri, A. gerencseriae, A. naeslundii |
| Infection pattern | Rare, usually polymicrobial; more severe if immunocompromised |
| Watch for | Arachnia propionica in canaliculitis (not Actinomyces, but mimics it) |
| Hazard / containment | Hazard Group 2 · Containment Level 2 |
The identification pathway tap each box
This is the logic the SMI flowchart encodes. Tap each step to see why it’s there — the goal is to be able to redraw this from memory.
Clinical specimen
Actinomyces selective agar
Metronidazole 10 mg/L + nalidixic acid 30 mg/L. A few species (incl. A. israelii) give classic molar-tooth/breadcrumb colonies here.
Fastidious anaerobic agar
Or equivalent agar without neomycin. Colonial appearance varies by species — majority are simply white/grey.
Gram stain
Looking for branching, beaded, filamentous or coccobacillary Gram-positive bacilli. Caution: easily over-decolourised → false Gram-negative.
Gram +ve, branching
Proceed to species-level identification (right-hand branch below).
Gram −ve
→ Not Actinomyces species. But: check decolourisation technique before accepting this at face value.
Molecular method
16S rRNA sequencing or NGS (ref. lab). Used for confirmation or when closely related species can’t be split by MALDI-TOF.
MALDI-TOF MS
Primary ID method. Limitations: database must be current; closely related species share protein profiles; old/chalky colonies reduce accuracy.
Conventional methods
Biochemical kits, indole, catalase. Not reliable used alone — confirm with taxonomic keys, not just the kit code.
Confirm species → report (refer to ref. lab if needed)
Why Gram stain lies to you signature interaction
Actinomyces is genuinely Gram-positive — but it is one of the easiest organisms to make look Gram-negative by mistake. Drag the slider to over-decolourise and watch what happens.
Correctly stained: Gram-POSITIVE
Gentle, correct techniqueExcessive — strips crystal violet
At a normal decolourisation step, crystal violet stays bound in the thick peptidoglycan wall — bacilli read out as deep violet, correctly Gram-positive.
Exam trap
Excess acetone or iodine/acetone during decolourisation strips crystal violet from the cell wall → Actinomyces appears falsely Gram-negative. If a “branching Gram-negative rod” report doesn’t fit the clinical picture, suspect technique before ruling out Actinomyces.
Two genuine look-alikes on Gram stain
Propionibacterium/Cutibacterium — pleomorphic bacilli that may appear to branch.
Nocardia — morphologically indistinguishable from Actinomyces on Gram stain alone (need other tests to separate them).
Nocardia — morphologically indistinguishable from Actinomyces on Gram stain alone (need other tests to separate them).
Strict anaerobe exceptions
Most Actinomyces species are facultative anaerobes. Three are strict anaerobes — worth knowing by name: A. israelii, A. gerencseriae, A. meyeri.
Colony morphology — the pattern, not the list generalised
The 90% rule
Don’t memorise 27 rows. Most Actinomyces colonies are white or grey, smooth, convex, with entire edges — visually unremarkable. Only a handful of species give the distinctive “textbook” appearance.
Tap a plate below to reveal the one detail worth remembering for that species.
Growth conditions — quick recall
| Parameter | Value |
|---|---|
| Optimum temperature | 35–37°C |
| Atmosphere | Anaerobic favoured; some grow aerobically or in air + 5–10% CO₂ |
| Visible colonies | 3–7 days |
| Full detection window | 10–14 days (don’t discard early!) |
| Primary isolation medium | Fastidious anaerobic agar (no neomycin — inhibitory) |
| Selective medium | Actinomyces selective agar: metronidazole 10 mg/L + nalidixic acid 30 mg/L |
| Broth enrichment | Rarely beneficial |
MALDI-TOF MS — 3 limitations flashcards
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Limitation
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Why it matters
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Conventional methods — the one number that’s testable
| Test | Result for Actinomyces |
|---|---|
| Spot indole | Negative (note: Propionibacterium/Cutibacterium acnes is indole-positive — a useful differentiator) |
| Catalase | Negative — except 5 species below |
Catalase-positive exceptions — memorise these 5
A. viscosus · A. neuii subsp. neuii · A. neuii subsp. anitratus · A. radicidentis · A. hominis
Biochemical kits alone are unreliable — databases can be outdated/incomplete, and reactions can be weak. Confirm with taxonomic keys.
Molecular methods
| Method | Use / limitation |
|---|---|
| 16S rRNA sequencing | Alternative/confirmatory. Struggles to separate A. naeslundii, A. viscosus, A. oris. Databases may contain erroneous top matches. |
| NGS | Mostly reference-lab only. |
Storage: sub-culture pure isolate into anaerobic broth. Referral: UK Anaerobe Reference Unit (UKARU), correctly packaged per transport regs.
Self-test mixed recall
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